Photochromic display device utilizing light valve activation

ABSTRACT

A photochromic display device in which a light valve projection system is utilized for activating the photochromic screen member to establish video information thereon. A uniform light source directs white light through the photochromic screen and is projected onto a viewing screen.

United States Patent Nix, Jr. et al.

[451 July 11, 1972 PHOTOCHROMIC DISPLAY DEVICE UTILIZING LIGHT VALVEACTIVATION Inventors: Lawrence A. Nix, Jr.; David L. Balthis,

both of Ellicott City, Md.

Assignee: Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed: Jan. 28, 1971 Appl.N0.: 110,423

0.8. CI ..l78/7.5 D, 178/DlG. 31, l78/7.87,

350/ i 60 P Int. Cl. ..H04n 5/74 Field of Search ..l78/DlG. 3 l, 7.5 D,7.85, 7.86,

[56] References Cited UNITED STATES PATENTS 3,495,034 2/1970 Arend etal. l 78/DlG. 3|

Primary ExaminerRobert L. Richardson ArrorneyF. H. Henson and C. F. Renz57] ABSTRACT A photochromic display device in which a light valveprojection system is utilized for activating the photochromic screenmember to establish video information thereon. A uniform light sourcedirects white light through the photochromic screen and is projectedonto a viewing screen.

4 Claims, 1 Drawing Figure PI-IOTOCIIROMIC DISPLAY DEVICE UTILIZINGLIGHT VALVE ACTIVATION BACKGROUND OF THE INVENTION This inventionrelates to large screen display devices capable of high speed electroniccontrol. The conventional cathode ray tube is suitable for applicationsup to screen diameters of 25 inches. Where larger display areas arerequired, a good solution to the problem has not been developed. Variousapproaches have been attempted in this area such as projection cathoderay tubes, pockel cell projection displays, and photochromic light valvesystems. These systems suffer from either poor writing speed, poorfrequency response, non-real time, poor light output, poor resolution,or various combinations of the above.

One projection system utilizes the pockel cell light valve. In thissystem, an electron beam is utilized to control the light transmissionfrom point to point in a crystal material such as ammonium or potassiumdihydrogen phosphate. This system appears to have acceptable resolutionbut has a very low efficiency as far as projection. The reason for thislimitation appears to be that the light source must be very accuratelycollimated prior to being directed onto the pockel cell. This requires avery high power light source.

Another system is that utilizing a photochromic material such asCyanamids Type 51-142 which exhibits the properties of transmission of acertain portion of the electromagnetic spectrum controlled by radiationin another portion of the electromagnetic spectrum. In currentphotochromic displays, the transmission is in the visible region of thespectrum and is controlled by radiation in the ultraviolet region of thespectrum. The current practice is to excite the photochromic materialwith a high energy cathode ray tube having a phosphor which emits in theultraviolet region. The major problem with the photochromic displaysystem is a requirement that the present photochromic materials requireabout 0.1 watt-second exposure per square centimeter to ultravioletlight in the active region of the spectrum. If a display havingreasonable resolution is to be obtained, several square centimeters ofthe photochromic materials must be employed. The present cathode raytube art will permit a power input-of about 0.6 watts per squarecentimeter without adverse affects on the ultraviolet phosphor. Thislimits the excitation energy available to about 75 or 80 milliwatts fora convention cathode ray tube or about 300 milliwatts from a direct viewstorage tube in that the phosphor is only about 7 percent efficient.These limitations result in a very low writing speed.

SUMMARY OF THE INVENTION This invention relates to a large area displayincorporating the best features of the pockel cell light valve systemand the photochromic display system in such a way as to provide animproved system and remove the objectionable features of the lightdisplay systems mentioned above. More particularly, the system utilizesa pockel light valve to control an ultraviolet light source which inturn excites a photochromic screen. The pockel light valve providesadequate energy to provide the desired writing speed on the photochromicmaterial. The photochromic projection permits the use of a wide aperturelight collimator since only the control power for the photochromicscreen need be handled by the pockel cell system. A relatively smalllight system is required to control the photochromic material in spiteof the poor collimination efficiency of the pockel cell system. A highef'ficiency projection light source may be utilized in combination withthe photochromic screen without the problem of requiring accuratecollimation.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of theinvention, reference may be had to the drawing which illustrates aprojection system of the preferred embodiment, exemplary of theinvention and incorporating the teachings of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, thereis illustrated a pockel cell drive unit 10 and a projection photochromicdisplay system 12. The pockel cell drive system 10 comprises a source oflight 14 which provides an emission in the ultraviolet range. A suitablelight source for this is a 100 watt mercury arc flood light. The lightemitted by the source 14 is collimated by a suitable optical means 16 toprovide a light beam consisting of rays substantially parallel to eachother. The light after passing through the collimator 16 is directedthrough a polarizer 18 which polarizes the light. The light is thendirected onto a pockel cell cathode ray tube 20. The pockel cell cathoderay tube 20 includes an evacuated envelope including a pockel cellscreen member 22 of a layer 23 of suitable material such as potassiumdihydrogen phosphate on an electrical conductive layer 25. This screen22 is scanned by a suitable electron gun 24 which produces an electricalfield pattern across the surfaces of the crystal 22 corresponding tovideo information applied to the electron gun 24. The light generated bythe source 14 and after passing through the polarizer 18, is rotated atan amount corresponding to the electric field impressed across thecrystal 22. The light after passing through the screen 22 is thendirected through an analyzer 30. The analyzer 30 is a device similar toa polarizer 18 and is oriented at to the polarizer 18 so that in theabsence of any activation of the crystal 22, no light will pass beyondthe analyzer 30. However, when the crystal 22 is activated by means ofexcitation by the electron gun 24 to impress a field across the screen22 in a point by point manner, the light passing through the analyzer 30is directed through a suitable focusing lens 32 and a dichroic mirroronto a photochromic cell 40. The cell 40 may be on any suitablephotochromic material such as type 51-142 manufactured by AmericanCyanamid Co., Stamford, Connecticut. The dichroic'mirror 34 has acoating of a suitable material such as that made by optical coatingLab., Inc., Santa Rosa, California which is transmissive in theultraviolet range but is reflective in most of the visible region. Amore complete description is found in Display System Engineering byLuxenbery & Kuehn and published by McGraw-l-lill Book Company, 1968.

The photochromic projection system 12 consists of a light source 50capable of emission of a wide spectral band within the visible regionsuch as an Xenon arc. This light is directed through a suitablecollimator 52 and is reflected by the dichroic mirror 34 onto thephotochromic film 40. In the absence of any activation energy from thedrive source 10, the photochromic cell 40 will be substantiallytransparent to the visible light from the source 50 and will passthrough the photochromic film into a projection lens 56 and onto aviewing screen 58. If light from the drive system 10 is directed ontothe photochromic film 40 from the drive source 10, then the film 40 willbe made more opaque to provide an image in which the image is projectedblack on white.

We claim as our invention:

1. A display system for display of an image of an object from videosignals comprising a first light source of a first wavelength, the lightfrom said first source transmitted along a first optical path forprojection of an image, a photochromic member positioned in said firstoptical path, a drive system for directing a scanning radiation beamover said photochromic screen to modify the opaqueness of saidphotochromic screen, said drive means comprising a second light sourceof a second wavelength, means for directing the light along a secondoptical path from said second source onto said photochromic screen,light valve control means positioned between said second light sourceand said light valve control means in said second optical pathcomprising an optically active screen member responsive to an electricfield applied thereacross and means to scan a raster over said opticallyactive material to modify the optical activity thereof in accordancewith a video signal applied to said scanning means.

2. The display system set forth in claim 1 in which light valve meanscomprises a cathode ray tube and said scanning means is an electronbeam, said optically active screen exhibiting the pockel effect, meansfor projecting the collimated light beam from said first sourcepolarized in a predetermined acceptance plane toward and through saidoptically active screen, said optically active screen comprised of aplate of 5 transparent non-conductive material having a layer oftransparent electrically conductive material thereto and being connectedto a potential source, said electron beam means scanning the surface ofsaid non-conductive material to create an electric field between saidsurface and said electrical conductive layer to vary the opticalproperties thereof in response to the value of an electrical fieldthereacross and thereby modify the polarized condition of said polarizedlight passing therethrough and a polarizing means having an acceptanceplane normal to the polarized condition of said light beam after passingsaid optically active screen when unexcited for intercepting the lightbeam from said second source after passing through said optically activescreen and passing only the portion of said light beam having a modifiedpolarization by said optically active screen.

3. The display system set forth in claim I in which said second lightsource emits light in the ultraviolet region.

4. The display system of claim 1 in which said photochromic screen isresponsive to said second wavelength by a change in its opaqueness tosaid first wavelength and is substantially non-responsive to said firstwavelength.

1. A display system for display of an image of an object from videosignals comprising a first light source of a first wavelength, the lightfrom said first source transmitted along a first optical path forprojection of an image, a photochromic member positioned in said firstoptical path, a drive system for directing a scanning radiation beamover said photochromic screen to modify the opaqueness of saidphotochromic screen, said drive means comprising a second light sourceof a second wavelength, means for directing the light along a secondoptical path from said second source onto said photochromic screen,light valve control means positioned between said second light sourceand said light valve control means in said second optical pathcomprising an optically active screen member responsive to an electricfield applied thereacross and means to scan a raster over said opticallyactive material to modify the optical activity thereof in accordancewith a video signal applied to said scanning means.
 2. The displaysystem set forth in claim 1 in which light valve means comprises acathode ray tube and said scanning means is an electron beam, saidoptically active screen exhibiting the pockel effect, means forprojecting the collimated light beam from said first source polarized ina predetermined acceptance plane toward and through said opticallyactive screen, said optically active screen comprised of a plate oftransparent non-conductive material having a layer of transparentelectrically conductive material thereto and being connected to apotential source, said electron beam means scanning the surface of saidnon-conductive material to create an electric field between said surfaceand said electrical conductive layer to vary the optical propertiesthereof in response to the value of an electrical field thereacross andthereby modify the polarized condition of said polarized light passingtherethrough and a polarizing means having an acceptance plane normal tothe polarized condition of said light beam after passing said opticallyactive screen when unexcited for intercepting the light beam from saidsecond source after passing through said optically active screen andpassing only the portion of said light beam having a modifiedpolarization by said optically active screen.
 3. The display system setforth in claim 1 in which said second light source emits light in theultraviolet region.
 4. The display system of claim 1 in which saidphotochromic screen is responsive to said second wavelength by a changein its opaqueness to said first wavelength and is substantiallynon-responsive to said first wavelength.